A standard hammer-drill chuck that is rotated about an axis of a drill spindle to rotate a drill bit about and reciprocate it along the axis has a chuck body securable to the spindle and formed centered on the axis with a screwthread. A collar rotatable but axially nondisplaceable on the chuck body is formed angularly equispaced about the axis with a plurality of angled jaw guides, although it is possible to form the guides on the chuck body and the screwthread on the collar for the same effect. Respective jaws in the guides have racks that mesh with the screwthread so that rotation of the collar on the chuck body about the axis in a tightening direction moves the jaws radially together and opposite rotation in a loosening direction moves the jaws radially apart. As described in U.S. Pat. No. 4,955,623, issued Sep. 11, 1990 a locking ring is provided that is limitedly angularly displaceable relative to the chuck body but nonrotatable thereon. In addition this ring is formed with teeth or other formations that mesh axially with complementary formations on the collar and the ring is axially displaceable on the chuck body between a back unlocked position with the formations disengaged and relative rotation of the ring and collar possible, and a front position with the formations in mesh and such relative rotation impossible. The function of this ring is to prevent accidental opening of the chuck.
As described in German patent document 3,406,668 the locking ring is axially behind the tightening ring, that is it must be pulled axially back to unlock the chuck. This is only possible when there is sufficient space at the back of the chuck. Thus this type of arrangement is not compact.